Technical Field of the Disclosure
The present embodiment relates in general to a method and apparatus for generating hydrogen. More specifically, the present disclosure relates to a method and system for continuously producing hydrogen gas, heat and aluminum oxide on-demand from solid aluminum using a water splitting process.
Description of the Related Art
Hydrogen can be generated by a variety of methods, including natural gas reforming, electrolysis, thermochemical reaction and photo catalytic methodologies. These methodologies produce carbon dioxide as a by-product, which requires a large amount of electrical energy which is expensive and has a large, negative environmental impact. These methods require solar energy with temperatures exceeding 1000 degrees Celsius, highly corrosive reactants and/or products and expensive reagents, complex nanostructured solids, and/or sacrificial oxidants or reductants other than water.
A number of variants of water split reaction used to produce hydrogen have been devised to overcome these problems. The water split reaction contemplates a fuel for splitting water into hydrogen and an oxide. In these reactions aluminum is used to generate hydrogen from water. Commonly, aluminum oxide compounds can be produced from bauxite ores by Bayer's process. In the water splitting process, the hydrogen is released as a gas and the oxygen combines with the aluminum to form the aluminum oxide compounds. The aluminum oxide compounds are produced as a protective oxide layer on the aluminum in contact with water at ambient temperature.
Aluminum has a tendency to be self-protecting by forming the aluminum oxide that inhibits reactions required for the formation of hydrogen and thus in some cases it is difficult, if not impossible, to use on a long term basis. Therefore, it has been accepted by those skilled in the art that the use of aluminum in a reaction with water to generate hydrogen gas requires that the protective oxide layer is efficiently and continuously removed, and that the reaction is kept at an elevated temperature.
In one prior art reference, U.S. Pat. No. 4,358,291, the inventors disclosed that if aluminum (Al) is dissolved in a liquid solution of gallium (Ga) or a liquid mixture of Ga and indium (In) at or near room temperature, then brought into contact with water, the Al in the liquid solution at the water interface would split water molecules (H2O) into hydrogen gas, alumina (Al2O3), and generate heat. This reaction will proceed until all elemental Al in the liquid solution is converted to alumina. The solid aluminum (Al) will dissolve in dry, air exposed liquid melts of gallium (Ga), Ga-indium (Ga—In), or Ga—In-tin (Ga—In—Sn) at or near room temperature up to the solubility limit of about 2-3 weight percent Al.
When inert solid Al is dissolved in liquid Ga melt the solute aluminum is no longer passivated with alumina, its native oxide. Hence, when water is in contact with aluminum saturated gallium melt, the aluminum atoms at an interface between the melt and water are free to split the water into hydrogen gas and alumina while generating heat. The gallium used is inert with respect to splitting water, and hence reusable.
One drawback of this approach is that if the Al that is dissolved in the liquid solution in a dry environment and reacted to completion in the presence of excess water, the liquid solution is now under-saturated with respect to Al. This means that the liquid could theoretically be saturated with additional Al. When a solid piece of Al (whose density is less than liquid Ga) is floated on top of an under-saturated liquid of Ga in the presence of excess water, the solid piece of Al will not dissolve into an under-saturated Ga, Ga—In, or Ga—In-tin (Sn) liquid at or near room temperature. Further, the solid Al does not dissolve in under-saturated liquid Ga in the presence of water due to the fact that there is a layer of water between the liquid Ga and the solid Al that forms a barrier layer of alumina that is thicker than the alumina layer that forms between Ga and Al in air. Attempts have been made to find other methods to cope with these problems. One method is to heat a mixture of solid Al and Ga (or Ga—In or Ga—In—Sn) in an inert container above the melting point of Al, and then return the melt mixture back to room temperature. However, this method requires the use of crucible materials that will not react with Al melts and causes difficulty to empirically find optimal cooling rates and composition that will render the mixture suitable for practical applications.
Another drawback of this approach is that if the liquid solution containing Al is cooled to the point of freezing into a solid solution, very little reaction will occur. This is because unlike the case for liquid solutions, where the Al atoms can continuously diffuse to water-solution interface and react until the Al has all reacted, Al atoms in the frozen solution cannot move to the interface. Hence, only those Al atoms at the frozen solution surface can react with water. Once the Al atoms at the frozen solution surface react with water, the reaction stops.
In light of the foregoing, there is a need for a method and system for continuously producing hydrogen gas, heat, and aluminum oxides on-demand from solid aluminum using water splitting techniques that avoid the inherent problems with current technologies. Such a method and system would need to be implemented on an inexpensive and economically viable basis. Such a method and system would provide an under-saturated gallium liquid melt that will not react with water when covered with water or exposed to air. Further, such a method and apparatus would continuously dissolve a solid-state Al or other liquid metals into the under-saturated Ga liquid melt and its alloys in the presence of water to enable the continuous generation of hydrogen gas; the continuous production of economically important oxides of Al or other liquid metals; and the continuous generation of heat. Such a method and system would not be passivated with alumina by continuously dissolving Al into liquid Ga in the present of excess water. Such a method and system would include a plurality of chambers in which the solid-state Al is continuously dissolved in the Ga liquid melt and Al saturated Ga melt is reacted with the water at water-liquid melt interface separately to split the water into hydrogen gas and aluminum oxide. Furthermore, such a method and system would include at least one means for separating water and gallium from the water-oxide mixture for the purpose of reusing the water and gallium during the process. Such a method and system would include at least one means for collecting the aluminum hydroxide and converting it into ultra-high purity (UHP) alumina. Finally, such a method and system would provide a continuous and economical conversion of the solid-state Al of any purity to on-demand UHP hydrogen and UHP alumina, using any kind of water. The present embodiment accomplishes these objectives.